On Mon, 30 Oct 2023, Vlastimil Babka wrote: > On 10/30/23 12:22, Mikulas Patocka wrote: > > > > > > On Mon, 30 Oct 2023, Vlastimil Babka wrote: > > > >> Ah, missed that. And the traces don't show that we would be waiting for > >> that. I'm starting to think the allocation itself is really not the issue > >> here. Also I don't think it deprives something else of large order pages, as > >> per the sysrq listing they still existed. > >> > >> What I rather suspect is what happens next to the allocated bio such that it > >> works well with order-0 or up to costly_order pages, but there's some > >> problem causing a deadlock if the bio contains larger pages than that? > > > > Yes. There are many "if (order > PAGE_ALLOC_COSTLY_ORDER)" branches in the > > memory allocation code and I suppose that one of them does something bad > > and triggers this bug. But I don't know which one. > > It's not what I meant. All the interesting branches for costly order in page > allocator/compaction only apply with __GFP_DIRECT_RECLAIM, so we can't be > hitting those here. > The traces I've seen suggest the allocation of the bio suceeded, and > problems arised only after it was submitted. > > I wouldn't even be surprised if the threshold for hitting the bug was not > exactly order > PAGE_ALLOC_COSTLY_ORDER but order > PAGE_ALLOC_COSTLY_ORDER > + 1 or + 2 (has that been tested?) or rather that there's no exact > threshold, but probability increases with order. It has been tested - Marek tested it with intial order == 4 and he hit the bug. With 3 and less he didn't. Mikulas
